In a collaborative meeting or teaching situation, it is sometimes useful to image the audience using a camera located at the position of the projection screen at which the audience is looking. For example, during a teleconference, where an audience sees an image of a person addressing the audience from a remote location, with a return video image of the audience provided to the speaker, it is desirable that the cameras that view the speaker and the audience be located as near to the respective screens as possible, to provide a realistic appearance. Otherwise, the image provided to the audience is of a speaker not looking directly at the audience, and the image provided to the speaker is of a group of people all looking away.
Placing a video camera at the location of a projection screen presents certain challenges, however. If the screen is illuminated by a front-projector, then light from the projector can interfere with the camera. That is, the field of view of the camera can encompass the light from the projector, allowing light from the projector to interfere with the image taken by the camera. At best this can produce a distracting light spot in the image taken by the camera, and at worst the light spot can substantially wash out the camera image. Although repositioning the projector or re-aiming the camera can in some cases allow one to remove the projector from the camera's field of view while still keeping the camera at the projection position, in many cases this is not possible. Additionally, a camera physically positioned in front of a front projection screen and within the projection region of the projector will cast a shadow on the screen, thus interfering with the projected image.
Some prior approaches to this situation have attempted to physically block or mask light from the projector onto the camera. However, there may be some situations where this cannot be done effectively, due to the geometry of the projector/camera setup and the room or other environment. Additionally, this approach still casts a shadow on the screen, thereby eliminating part of the image, and still requires careful alignment to place the camera in the blanked out region. In cases where the camera is placed behind a transparent or semi-transparent front projection screen, scatter from the screen can still cause image artifacts even with shadowing or masking of the camera position.